The efficiency of a steam power plant increases with increasing temperature of the steam produced in the steam boiler. However, upper limits for the temperature of the boiler pipe material and of the turbine upon which the steam acts must not be exceeded. The more precisely the temperature can be kept to the target value, then the closer the target value can be kept to the admissible temperature limit, i.e. the higher the efficiency level that can be achieved during operation of the generating plant.
Superheating of the steam in the boiler is achieved in that the steam is fed through the heated bank of pipes in several stages—the superheating stages. Control of the steam temperature is carried out by injecting water into the steam tube before the superheating stage via suitable injection valves. The superheaters with their very large masses of iron exhibit very sluggish behavior. Adjustment of the injection valve has an effect on the temperature being controlled only after several minutes. The time delay is not constant, but depends on the momentary steam mass flow rate. Furthermore, the temperature to be controlled is strongly influenced by numerous disturbances such as load changes, soot build-up in the boiler, changes of fuel, etc. For these reasons, precise temperature control is difficult to achieve.
Cascade control, in which two nested PI control loops are built up is known for solving this problem. An outer, slow PI controller controls the temperature at the superheater exit and outputs a target value for the temperature at the superheater entry—i.e. following the injection. The temperature at the superheater entry is adjusted by an inner, rapid PI controller which adjusts the injection valve. Disturbances of the steam temperature at the entry point of the injection can thus be rapidly corrected. The disadvantage of this concept is that disturbances which affect the superheater itself can only be corrected in the outer, slow circuit—i.e. with low control quality.